Propelled by emerging smart mobile devices and applications, the mobile data traffic is projected to increase 13-fold between 2012 and 2017. To accommodate the targeted data rate requirements, such as 1-Gb/s per cell site in the Long Term Evolution-Advanced (LTE-A) standard, a large number of small cells that reuse available spectrum will be needed to provide economical high-speed mobile traffic delivery. As a result, the ability to cost-efficiently support high data rates with low latency are among the essential requirements for fourth-generation and beyond (4+G) mobile backhaul (MBH) systems. From the technical perspective, synchronization is a fundamental requirement to realize all of these key features. Moreover, as data rates and latency requirements continue to rise, legacy synchronization techniques may mandate careful re-design to keep up with the dramatic traffic and architectural changes of 4+G mobile backhaul systems.
In legacy MBH systems, base stations are typically connected to T1/E1 copper lines and merged into the synchronous optical network/synchronous digital hierarchy (SONET/SDH) in which the timing information from the primary reference clock (PRC) is inherently built into the transport layer, and slave clocks at base stations are traceable to the PRC. However, as MBH networks migrated from the legacy time division multiplexing (TDM)-based architecture to packet switched operation, new synchronization techniques and protocols were needed and proposed to distribute synchronization information. The two primary methods in this space are Synchronous Ethernet (SyncE), as defined by the ITU-T in Study Group 15, Question 13 (Q13/15) and the Precision Time Protocol (PTP) as defined by the IEEE 1588v2 standard. SyncE provides accurate frequency distribution at the physical layer, but is not protocol-transparent; it requires that each node in the network be SyncE enabled, which might not suit all deployment scenarios. The PTP protocol distributes frequency and time synchronization via timing information carried by the packets, yet also needs customized hardware for timing measurements, and suffers from traffic-dependent synchronization accuracy. It has thus been suggested to combine SyncE and PTP to ensure end-to-end high accuracy. However, this approach involves both physical and packet layer processing, and can also increase processing complexity and delay and pose a challenge to satisfying low-latency requirements of future MBH systems, particularly as data rate requirements for backhaul systems increase beyond 10 Gb/s per-channel.
Optical MBH based on intensity modulation/direct detection (IMDD) orthogonal frequency division multiple access (OFDMA) techniques for high-speed, low latency optical MBH to hundreds of cells per fiber have been demonstrated. However, to practically implement OFDMA-based optical MBH, novel low-latency synchronization techniques are needed. Specifically, in addition to system-level synchronization, symbol-level synchronization in both the OFDMA transmitter and receiver is also needed for accurate real-time transmission. In optical OFDM-based access networks where latency is not a critical issue, the timing information can be embedded in the received OFDM signal and recovered through intensive digital signal processing (DSP). Recently, digital signal processing (DSP)-free synchronous clock distribution based on out-of-band and in-band clock transmissions and electrical filtering have shown for single-wavelength directly-detected optical OFDM systems. However, the frequency and time alignment performance with respect to 4+G mobile backhaul requirements was not evaluated. Moreover, only fixed-rate rather than flexible clocks were considered in conventional systems, and pre-scalers were needed to derive both low-frequency square wave clocks for digital circuits and high-frequency sine wave clocks for sensitive mixed-signal circuits, such as digital-to-analog converters (DAC) and analog-to-digital converters (ADC).